In the container-making industry, containers are typically manufactured in at least two parts: a container body and at least one container end. The container body may be drawn and ironed such that only a single container end is required (two-piece container) or the container body may be formed by rolling a stamped sheet into cylindrical form and welding the seam such that two container ends are required (three-piece container). Regardless of the particular container structure, after the container is filled, container ends are typically double-seamed to the open end. More recently, the open end of metal containers has been necked prior to end piece connection. By reducing the diameter at the open end of the container body, the amount of end piece material can be decreased to lower packaging costs, and containers can be stacked more readily to accommodate storage, handling and display.
Numerous techniques for necking the open end of a container body have been developed. Such techniques generally entail the use of external dies and/or rollers which act upon the outside of a container body. As used herein, a "die-necking" operation is an operation wherein a cylindrical container body and inward reducing die are axially aligned and opposingly advanced to force an open end of the container body through the reducing die. Due to the high compressive forces imparted to the container bodies in die-necking operations, only a relatively small reduction in diameter per operation can be achieved without sidewall buckling or crumpling. As such, several successive die-necking operations are often necessary to achieve a desired diameter reduction.
In necking processes utilizing external rollers, one or more rollers contact the sidewall of a rotating container body near an open end thereof and are driven radially inward. A cylindrical member is internally and rotatably disposed at the open end of the container body to support the open end during such processes. In some known processes, no internal support is provided in opposing relation to the inward progression of an external forming roller, thereby resulting in process control problems which, in practice, limit the degree of inward necking. Further, in such known roll-forming processes, the configuration and relative positioning of the external roller and interfacing cylindrical member cause the open end of the container body to be drawn through an extremely sharp radius therebetween (i.e., approaching a 90.degree. bend) to form a finished flange and generate a risk that metal slivers will be created within the container body. Such contemporaneous flange forming and production risk also limit, in practice, the degree of realizable inward necking.
Recently, a novel necking technique, known as "spin-flow forming" and described in U.S. Pat. Nos. 4,563,887 and 4,781,047, has been developed in which two internal members are provided to support and thereby control a rotating container body as an opposing external roller progresses radially inwardly and axially to neck the container, thereby allowing for significant increase in the degree of inward necking that, in practice, can be realized in a single process step. More recently, it was discovered that substantial benefits could be realized by the combinative use of die-necking and spin-flow forming operations. By die-necking prior to spin-flow forming, plug diameter variations in container bodies are substantially reduced prior to spin-flow forming, thereby reducing the likelihood of container body failure during spin-flow forming operations and increasing container uniformity upon spin-flow forming. Such combinative use of die-necking and spin-flow forming operations is disclosed in U.S. Pat. No. 5,138,858.
While the combinative utilization of die-necking and spin-flow forming has reduced the likelihood of container body failure during spin-flow forming operations and has increased container uniformity, container bodies undergoing spin-flow forming are still susceptible to "krinkling" failure under certain situations. Krinkling is caused by torsional forces on the container body (e.g., the container sidewall) exceeding the torsional strength thereof. A krinkling failure typically manifests itself as a "z-shaped" nonuniformity in the sidewall of the container body immediately below the shoulder radius. Such failures due to krinkling have become increasingly problematic with decreasing container sidewall thicknesses and increasing production speeds.
Consequently, it is an object of the present invention to increase the efficiency of the spin-flow forming operation. It is a related object of the present invention to improve the spin-flow forming process by decreasing the occurrence of sidewall failure due to krinkling and/or by allowing increased production speeds.